When a single snowflake falls peacefully atop a Sierra peak, it begins a turbulent journey to help quench the thirst of a drought-stricken state.

In most years, Sierra snow provides a third of California's water supply. But it is by far the least reliable portion — and now, after three years of historically low snowfall, tensions are soaring over how we share the shrinking bounty of this great frozen reservoir.

Today, on the cusp of a long, dry summer, we follow the melting snow — and meet its dependents — along one of its many routes from remote peaks to thriving communities around the Golden State.

Snow-covered mountains of the Tuolumne River watershed are photographed on Wednesday, May 7, 2014, in Yosemite National Park, Calif. After three years of low snowfall, tensions are soaring over the use and rights to California's limited water supply. Like almost every river in California, the Tuolumne is oversubscribed: fishermen, kayakers, farmers, and electric plants lay claim to the water before it flows into our faucets. (Aric Crabb/Bay Area News Group)

As our snowmelt travels the 300-mile path from Yosemite's Mount Dana to the sea, it meanders through the Tuolumne River watershed, past hydropower plants and nurseries, wildlife refuges and chemical plants, vineyards and the San Francisco Bay Area, where it provides water for millions of people.

Each of these water users, linked by a reliance on this fragile resource, has a legal right to some of the flow — and a growing need to insist on those claims.

The trouble is, in an average year, five times more water is committed than flows through all the state's rivers and streams combined, according to new research by University of California scientists. (Chart: Overdrawing California's rivers.) The state's population growth will further boost this demand. And climate change predictions suggest our water supply will only continue to diminish.

The Tuolumne's story is typical of so many other stressed rivers in the spacious and sunburned state, where a century-old system of water rights divvies up a precious resource for California's vibrant 21st century economy and 38 million residents.

“We've created a false sense that there is sufficient water to meet everyone's needs,” said Theodore Grantham, a UC Davis watershed researcher who co-wrote the new analysis on the state's water demands.

“Now, there's a crisis. But it is also an opportunity. Crisis breeds innovation.”

Trickling into the Tuolumne, our journey begins

Yosemite National Park, birthplace of Tuolumne River (Mile 0)

Drifting down from pewter skies, our snowflake lands here: the volcanic shoulder of Yosemite's 13,061-foot-high Mount Dana, one of the highest peaks in the Tuolumne River watershed.

Stationed on the mountain, a tiny sensor gathers snowfall data for Frank Gehrke, the don of Sierra snowpack. Several times a year, the Department of Water Resources engineer drives to Tioga Pass, straps on his cross-country skis and trudges in the thin air to check the accumulation.

The season's most important tally, taken April 1, revealed dismaying news, despite late March snow: only 16.8 inches of snowpack, about 40 percent of the historic annual average of 42 inches. Overall, the Sierra snowpack was 32 percent of average — the lowest level on April 1 since 1988. Gehrke, 66, uses his measurements as the basis for models and forecasts that help allocate the year's water supplies — a report that he says “drives the whole economy.”

Miles downstream, in a pine-scented forest, Yosemite hydrologist Jim Roche tests the cold snowmelt after it has trickled down granite, drained into a magnificent canyon and emptied into the pristine Tuolumne.

One day last month, he measured temperature and clarity as he crouched over a small stream above Hetch Hetchy Reservoir, then descended 3 miles to a lush wetland, where he measured the water's velocity. He found the water flowing at 81 cubic feet per second, compared with typical late-May flows of 300 to 1,000 cubic feet per second. A colleague's measurements also worry him: The ice pack on Yosemite's nearby Lyell Glacier has decreased by 60 percent since 1900.

Here in the deep wilderness, the findings of these scientists influence many urban decisions: Does our water need to be filtered? What will it cost? Is there enough so we can water our lawns and wash our cars?

Following the meandering Tuolumne River shows the answer is complicated. When there is not enough water to supply everyone's needs, a generations-old pecking order of claims determines who gets the most. But this year, almost everyone will get less than they want.

At Hetch Hetchy, signs of an alarming shortage

Hetch Hetchy Reservoir (Mile 35)Moccasin Power Station (Mile 80)

The wild river, clear as chilled gin, is trapped at Hetch Hetchy Reservoir, built in the 1920s to provide water security to fire- and earthquake-ravaged San Francisco.

In a normal year, so much water flows from the snowpack that the reservoir is filled, and emptied, three times. This year, the reservoir is unlikely to fill even once.

That's a big problem at this critical junction, where three different groups have rights to the water.

Some belongs to two farm irrigation districts downstream. Other water is devoted to wildlife.

But Hetch Hetchy's central purpose is collecting water for 2.6 million residents of the San Francisco Bay Area. From here, the water passes unseen through granite into the Canyon Power Tunnel, before it is delivered 167 miles — in and out of reservoirs, beneath rolling foothills and under the San Joaquin Valley floor — to an increasingly congested and thirsty Bay Area.

“We have enough water for another two years as long as we conserve,” said Steve Ritchie, an assistant general manager with San Francisco's PUC. “The thing is, what if there's a three-year, or four, or biblical drought? You have to be prepared to deal with that.”

Even though it's first along the river, the San Francisco Public Utilities Commission, which manages Hetch Hetchy, can't just take all the water it wants. It is required to release water downstream so irrigation districts in Modesto and Turlock with older claims on the river get the water they are guaranteed.

The water here plays yet another important urban role: energy generation.

Thirty miles from Hetch Hetchy, it plunges down a 1,000-foot ridge at the tiny town of Moccasin, an old stagecoach stop, where it is funneled into turbines to generate 1.6 billion kilowatt hours of power a year.

As a low-cost, no-pollution energy generator, hydroelectricity is a coveted power source. The Moccasin turbines generate so much energy, with such regularity, that they power much of San Francisco — from Muni buses and schools to the airport and port. Electricity is also sold to irrigation districts and on the open market.

This year's low flow, however, means less electricity, Ritchie said. Normally, these turbines produce $7.7 million worth of energy to sell to irrigation districts and another $8.3 million on the open energy market. This year, there's enough to power the city, but the commission expects to sell only $1 million to $2 million to irrigation districts — and nothing else.

Uncharted territory for region's tourism industry

Don Pedro Marina (Mile 95)

Where rugged mountains give way to ruddy cliffs, the springtime pulse of the river downstream from Hetch Hetchy is a joy to those who raft, paddle kayaks or float in houseboats.

The sporadic bursts of water mean that three Tuolumne County kayak companies can continue to earn a living here, amid blue oaks and bald eagles — part of a bustling tourism business that also includes camping, fishing and visits to Gold Rush towns.

But the overall flow is too weak to fill Don Pedro Lake — the main holding tank for several hundred square miles of productive Central Valley farmland and Modesto's drinking water. What enters the reservoir is going out even faster.

Water levels are plummeting 6 to 8 inches every day; by fall, the 830-foot water level is expected to drop to 678 feet.

To protect his valuable houseboats, with their Jacuzzis, wet bars, dishwashers and TVs, the reservoir's resort manager is consolidating three marinas to the one on the deepest parts of Don Pedro, spending $30,000 on electric cable and thousands more for extra buoys, longer lines, boat slips and anchors.

Campers are rethinking plans, with reservations lower than they have been in the last six years, said Jim Smith, the marina's general manager.

“This is totally uncharted territory for me,” said Smith, 56, his face creased and weary from working 10-hour days, seven days a week. “You adapt, because Mother Nature is not going to adapt to us.”

Falling water levels do offer new adventures, such as exploring the now-exposed remnants of the historic Eagle-Shawmut gold mine. Hikers are finding specks of gold in dry creek beds, as well as lost cell phones and sunglasses. However, the lure of those activities might not pay Smith's bills.

Downriver, thirsty farms claim historic rights

Hickman (Mile 120)

The water in Don Pedro Lake was promised long ago to entrepreneurial farmers who built the dam and created the Turlock and Modesto irrigation districts. About half of the 1.7 million acre feet of water captured by the Tuolumne River watershed goes to their farms. San Francisco's PUC gets about 12 percent, and the rest goes downriver.

These mighty districts — whose canals extend for more than 400 miles — sit atop the Tuolumne River's human pecking order because they made their claim in 1887, under a water rights system that emerged with the early settlement of California known as “first in time, first in right.” Their access to water trumps San Francisco's. But they rank below the rights of wildlife, which are protected by federal law — a major source of conflict in the region's age-old fight for water.

Successful use of water has increased the productivity of the farms. At Frantz Nursery, which has 150 employees and has grown from 5 to 40 acres, millions of plants are shipped to places ranging from Costco warehouse stores to Larry Ellison's Woodside estate.

“Were it not for the Tuolumne River, our business would not exist today as it is,” said Michael Frantz, 37, a second-generation nurseryman and board member of the Turlock Irrigation District. “It is the economic driver that enables us to be here. This entire region relies on the surface water diverted from the Tuolumne.”

But the drought dries up even this long-held guarantee of water — a fact that worries Frantz, who will get only half of his normal allotment of water this summer, as the irrigation district tries to conserve.

“I love this river,” said Frantz, who as a boy built campfires, dug up clams and fished along its banks, then learned how to shovel, repot seedlings and fix irrigation pipes. Now, he has invested $1 million in a system that traps, treats and reapplies irrigation water to the vast rows of plant containers.

“We're nervous,” about reductions, he said. “We've done the math and we think we are going to be fine. But it's going to be close. We're not applying any more water than we have to.”

Slowly, nature recovering from human intrusion

San Joaquin River National Wildlife Refuge (Mile 175)

Where the clean and graceful Tuolumne merges with the muddy San Joaquin River, creating a vast Delta floodplain, wildlife biologist Eric Hopson is helping reclaim a damaged landscape.

About 95 percent of the San Joaquin Valley's riverfront wetlands have been lost to farming and urban development in the past century. Gone are the vast flocks of migrating birds that once darkened their skies.

But this precious confluence west of Modesto, surrendered by farmers who tired of constant flooding, is beginning to return to the wild.

“It's like a small museum — a relic habitat,” said Hopson, who lives on the refuge with his wife.

He strides through hip-high brambles in the 7,000-acre San Joaquin River National Wildlife Refuge, looking for signs of the riparian brush rabbit and listening for the “cheedle-cheedle-chee” call of the least Bell's vireo.

Both species — absent for more than 40 years — again have been sighted along the river banks.

Here, the water is carving new channels through old fields, where volunteers have planted thousands of native trees.

“The river is reclaiming itself,” said Hopson, the refuge's assistant manager.

But suburban sprawl casts a long shadow over its future. The refuge is an isolated island of wilderness surrounded by an encroaching sea of Central Valley development.

And while nature can rebound after a drought, Hopson worries that our efforts to manage the extreme flows of the river could threaten this biological success story.

“Lack of water is hard on vegetation and wildlife. But it can adapt, over millennia,” he said.

“But if the cycles go from raging floods one year, to droughts like we've had over the past three years, that's hard,” he said.

“Boom and bust, flood to drought — that does not bode well for wildlife.”

Closer to the bay, industry frets over unstable supply

There's something special about the Pittsburg location of giant manufacturer Dow AgroSciences: Unfettered access to water. As a waterfront property owner, Dow has the right to pump water directly from the Tuolumne-fed San Joaquin River.

But for decades, the chemical plant — located along the New York Slough, near the broad, slack mouth of Suisun Bay — has relied on a less expensive strategy: Buying water from the local provider, the Contra Costa Water District, which gets its supply from the federal government's Central Valley Project. It's shipped via aqueduct from the Tuolumne-fed Delta.

This year's drought, however, has forced some rethinking about this longtime relationship.

Dow worries about its future, because the federal government is curbing supplies. A company that had its start as the 40-acre Great Western Electrochemical Co. now needs at least 300 million gallons a year — 500 to 600 gallons a minute — to fill a global hunger for fumigants, antimicrobials and pesticides.

It's one of several major industrial manufacturers in the area that rely on massive amounts of water, such as the steel-finishing plant USS-POSCO, owned by U.S. Steel and Korea, and refinery Tesoro Corp., which markets Shell, Arco and other fuels.

The largest chemical plant west of the Mississippi, with 350 employees, Dow uses Delta water to fill five-story-tall cooling towers and the “scrubbers” used to clean gas emissions before they are vented from smokestacks. It also needs super-clean water for direct use in its products.

“Water, like energy, is a large part of successful manufacturing,” said Randy Fischback, Dow's spokesman.

So Dow is studying how to extract and purify water directly from the slough — a possibility thanks to the company's rights to the river water. Environmentalists are bound to raise concerns. But so-called “riparian rights” are among the most protected water rights in the state, said Timothy Moran of the State Water Board. Although Dow would be required to file its intentions with the state, it would not need a permit.

“It helps us control our own destiny,” Fischback said.

In Livermore, making wine from ever-dwindling water

Livermore Valley (Mile 230*)* — Water delivered from State Water Project

Like so much of Northern California's surface water, the snowmelt joins the giant junction of the San Joaquin-Sacramento River Delta, the largest estuary on the West Coast and a vital water supply for 25 million people and more than 3 million acres of farmland.

But Californians have been fighting over that water, much of it diverted by state and federal canal systems, since the 1930s.

When there isn't enough fresh runoff into the Delta, it turns salty and polluted. The perilous future of the Delta is pitting farmers against fish and turning this critical intersection into one of California's biggest environmental conflicts.

Gov. Jerry Brown backs a contentious $25 billion plan to re-engineer the estuary by diverting water south around the Delta through two massive tunnels.

The impasse frustrates fourth-generation Livermore winemaker Phil Wente, who is entrusted with the survival of a vineyard that has been in his family since 1883 and has watched the Delta debate rage for years.

Since its founding, Wente Vineyards has grown from 47 to 3,000 acres, adding a restaurant, concert performances and championship golf course. It attracts tourists and boosts local real estate values.

But the unreliability of water puts all that at risk. Wente, along with other Valley vineyards, has been told to expect only three-quarters of his usual water allotment this year. It could be far worse: For the first time in its 54-year history, the State Water Project is providing no water to urban residents or farmers this year. But Wente's water provider, the Zone 7 Water Agency, is filling the gap by pumping groundwater and tapping reserves. Wente uses drip irrigation and other conservation measures to stretch his supply.

Still, he's worried about the future. The upstream diversion of Tuolumne water to urban areas is starving the Delta, he said.

“My concern is that if we're never going to fix the Delta, & we should move on and find another way to create a reliable water system” he said.

A diminishing catch, and a disappearing way of life

Berkeley Marina (Mile 290)

Out to sea beyond his berth at Berkeley Marina, where stinging winds whip the Pacific into a froth of whitecaps, fisherman Mike Hudson has seen large sharks and battled treacherous 16-foot seas.

But what really scares him is the shrinking king salmon population — and the loss of the fresh-flowing water needed to support his ancient way of life. Three decades ago, California had 4,000 licensed salmon boats; now, the number has fallen to 1,200, only half of which go out every year.

The salmon he seeks take a journey much like the snowflake, hatching in cold waters and then migrating to the ocean. But as the Tuolumne and other rivers dwindle, there is less water to flush the young fish out to the Delta, which already is a tough neighborhood for baby salmon to navigate. Moreover, adults have a hard time returning up the river to spawn — due to all the dammed reservoirs.

Once, millions of salmon swam off California's coast. This year's population is estimated at 630,000. The drought has triggered emergency measures to save them from extinction, with the state shipping baby salmon, by truck, because there's not enough water for them to swim.

And fishers like Hudson are disappearing with the diminishing catch. Three years ago, when populations dropped so low that fishing was banned, “some guys threw in the towel,” he said. “You cry a lot.”

“The rivers are just shadows of what they used to be,” said Hudson, who fishes 75 miles offshore between Monterey and Bodega bays and sells his catch at the Berkeley, El Cerrito and Kensington farmers' markets.

“Spring runoff is a conveyor belt of precious cargo — baby salmon — to the ocean. Now we don't have the same type of runoff,” he said.

Reliable water? Residents ‘absolutely take it for granted'

The snowmelt simmers in San Carlos, on a cooktop in Suzanne Emerson's kitchen, where it melts paraffin for daughters Jessica and Katie's Girl Scout project.

This Tuolumne River water didn't make its way here by meandering past the houseboats at Don Pedro Lake, the nursery in Hickman or the willows in the San Joaquin River National Wildlife Refuge. It came directly from Hetch Hetchy, 160 miles east, through a network of pipes and tunnels to the Crystal Springs Reservoir west of San Mateo.

It's the water that rinses the Emerson girls' freshly brushed teeth, quenches their thirst and waters the small yard, where they climb into a tent to celebrate summer with friends during a backyard slumber party.

“I am glad we have a good source of water here that we don't need to worry about,” said Emerson, 46, an environmental consultant and co-founder of the volunteer group San Carlos Green.

“There's no fear of contamination, or an ‘off' flavor,” she said. “And it gets here through pipes that go all over the place.”

Still, in this third year of drought, like most municipal water providers, the San Francisco PUC has been asking for voluntary reductions but is now considering mandatory restrictions. San Carlos, for instance, has asked its residents to cut water use by 20 percent.

To conserve, the Emerson family has swapped out 1940s-era toilets, shower heads and faucets with new, more efficient ones. Water use bumped up slightly when preteens Jessica and Katie started daily showers. Then it fell a notch after the yard was relandscaped.

The Emerson family of four uses 125 to 175 gallons of water a day — far above the 5 gallons of water used by the average African family, but far below California's per-person average of 196 gallons a day.

If they lived, instead, in a developing country, Emerson might walk an average of 4 miles a day to collect water — balancing on her head a bucket weighing about 40 pounds.

At a kitchen table with neighbor Christine Meeks, the women's conversation turns to the role that pristine Sierra snow plays in their comfortable Bay Area lives — and the engineering marvel that delivers it, almost invisibly, to their faucets.

“We absolutely take it for granted,” said Meeks.

In this historic drought, however, with so little water to share, nothing is guaranteed anymore.

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Major California cities

Californians in six major cities live far from their water sources, with major aqueducts carrying the precious liquid to them. Click on a city to see which aqueduct delivers its water.

California major cities and their water sourceSan FranciscoSan JoseLos AngelesSan DiegoBakersfieldOakland

Data: Water Education Foundation
Qin Chen / San Jose Mercury News

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A panel of experts will discuss the California drought - and the future of Bay Area water supply and its cost - from 6:30 p.m. to 8:30 p.m. July 17 at Lucie Stern Community Center, 1305 Middlefield Road, Palo Alto. Reservations for the free event can be made at www.surveymonkey.com/s/July17drought. Information available at mreynolds@bayareanewsgroup.com.

Who has water rights?

In California, we don’t own water. Rather, we just have the right to use it.

There is a pecking order to these rights, regulated by the State Water Board, which determines who can take water when there’s not enough to go around.

Riparian rights: The oldest and most senior of rights, dating back to English common law, given to owners of waterfront property, who must share water with other waterfront owners.

Appropriative “senior” rights: Next in line, these are given to users who aren't connected to waterfront property but who made a claim on a river, stream or lake before 1914 under a long-held Western tradition of “first in time, first in right.” Not subject to permit or license.

Appropriative “junior” rights: Given to users who aren't connected to waterfront property and filed after 1914. Must obtain permit and license from state.